1. Field of the Invention
The present invention relates to a tyre for a motor car. In particular, a high-performance motor car.
2. Description of the Related Art
Motor vehicle tyres having a tread provided with blocks delimited by circumferential grooves extending in a substantially longitudinal direction and by transverse grooves extending in a substantially axial direction are known. The blocks resulting from the intersection of said grooves are formed in various suitably designed shapes and are arranged in adjacent circumferential rows, each of which is located between two successive circumferential grooves.
The circumferential grooves may influence the directional and travel stability properties of the tyre in relation to the lateral (slip) thrusts directed parallel to the rotation axis of the tyre.
The transverse grooves, in turn, may influence the tractional properties of the tyre, namely its capacity to transmit efficiently to the road surface the tangential thrusts parallel to the direction of travel during acceleration and braking of the motor vehicle.
The circumferential and transverse grooves may also influence the draining of the water in the area making contact with the road surface (footprint area) during travel on a wet road surface.
In any case, the presence of transverse and circumferential grooves influences the rolling noise of the tyre. In fact, one of the main causes of noisiness is the continuous succession of impacts of the edges of the blocks on the road surface.
A further cause of noisiness consists in dragging of the blocks on the road surface when they enter and leave the area of contact with the road surface. This dragging is due essentially to the deformation of the tread both when the tyre is flattened against the road surface and when it recovers its inflated condition, as it leaves the area of contact with the road surface.
The deformations of the tread when coming into contact with and leaving the area of contact with the road surface also produce a cyclical volumetric variation of the grooves which delimit the blocks and a consequent cyclical action of compression and expansion of the air trapped inside the grooves. These phenomena of compression and expansion of the air increase the rolling noise of the tyre.
Various measures which tend to limit the rolling noise of a tyre are known. One of these consists in providing the blocks with different longitudinal dimensions, by adopting two or more different pitch values distributed in circumferential succession—called “pitch sequence”—such as to provide the maximum possible lack of uniformity over the circumferential extension of the tread. The object is to distribute the acoustic energy due to the impacts and dragging of the blocks over a wide spectrum of frequencies, thus avoiding concentrating it in a specific frequency and producing bothersome noise.
The noisiness of a tyre, measured in accordance with the standard ISO/TC 31/SC No. 623, is considered unacceptable when it exceeds the following limits:
tire width ≦14572 dB(A)tire width >145 and ≦16573 dB(A)tire width >165 and ≦18574 dB(A)tire width >185 and ≦21575 dB(A)tire width >21576 dB(A)
The noisiness of tyres is a problem which is difficult to solve because some measures which tend to reduce it adversely affect the directional, tractional and water drainage properties.
For example, in order to improve the water drainage properties, the transverse grooves should be steep, namely they should have a small inclination with respect to the circumferential grooves. On the other hand, in order to improve the uniformity of travel and the response time on a dry road, the transverse channels should have a large inclination with respect to the circumferential grooves, namely should be substantially perpendicular to the longitudinal grooves. However, transverse grooves which are very inclined worsen the rolling noise of the tyre.
Moreover, sounds with a frequency of less than 1500 Hz, or more particularly less than 1000 Hz, are much more noticeable inside a car than outside, while sounds with a frequency greater than 1500 Hz are much more noticeable outside a car.
Therefore, it is not possible to obtain a low noise level both inside and outside the car and it is difficult to achieve in each case the best possible compromise.
EP-812,709 describes a tyre having a tread comprising two regions at least one of which is provided with a plurality of inclined main grooves, each of which comprises a segment with a steep inclination and a segment with a slight inclination. A lateral band of the two regions has an auxiliary groove with a steep inclination communicating with two adjacent, inclined, main grooves and an auxiliary groove with a slight inclination, situated between the two adjacent, inclined, main grooves.
In a first embodiment, the inclined main grooves extend from a circumferential groove close to the equatorial plane of the tyre, while in a second embodiment they have a blind bottom.
In a third embodiment, the steeply inclined segments of two adjacent, inclined, main grooves are joined by a thin transverse groove.
EP-867,310 describes a tyre comprising blocks formed in a tread portion by means of a plurality of circumferential grooves and a plurality of directionally inclined grooves. At least some of the directionally inclined grooves extend from a circumferential groove close to the equatorial plane of the tyre and extend towards one end of the area of the tread making contact with the ground. Each of the blocks has an angled portion which forms an acute angle of 10°-60° defined by a circumferential groove and a directionally inclined groove. The surface of the angled portion of the block is chamfered over a distance of 10-30 mm from a tapered end thereof in a longitudinal direction so as to vary gradually towards a portion of larger width.
In the tread of both the abovementioned documents, each inclined groove delimits a block and the adjacent one located circumferentially alongside. Namely, each directionally inclined groove separates two adjacent consecutive blocks and is common to them.
The result is that both the abovementioned treads have circumferential rows of sickle-shaped adjacent blocks which are separated only by a pitch equal to the sum of the length, in the circumferential direction, of a block and of the adjacent inclined groove.